The A4 has a dedicated hardware video decoder in the form of a PowerVR VXD375. The Hummingbird can do it in hardware too, but with its more generalized GPU and/or SIMD units. Both can pump out the video, but the VXD375 lets Apple do it using very little battery. The iPhone 4 gets IIRC around 50% more video play time compared to a Galaxy, even though iPhone 4 has a smaller battery and more pixels to push (higher-res screen).
I’ve tried google and my usual tech sites and I can’t find anything that says the A4 has different or better video decoders than the hummingbird. If you have information please direct me to it, I would find it interesting.